Former slave Francisco Rodrigues dos Santos on the piece of land where he lives and farms in Monsenhor Gil, Piauí state, north-east Brazil.
Photograph: Mario Tama/Getty Images

The Brazilian government has been accused of reducing its ability to protect workers from slave-like labour conditions after abruptly changing the rules. Campaigners, commentators and prosecutors said the move was a “social regression” aimed at buying the support of a powerful agribusiness lobby ahead of a crucial vote in congress that could cost President Michel Temer his mandate.

A government directive by the ministry of labour published on Monday redefined what the government defines as “slave-like work” – even though Brazil’s efforts to stop abusive labour conditions were praised as recently as last year by the United Nations.

The ministry will no longer automatically publish its “dirty list” of employers whose workers were kept under abusive conditions. Instead it will only appear if the current minister decides to make it public. Many of the employers on the list are farmers.

“For us it will be a real regression in the battle against slave labour. It will make the definition harder and make inclusion on the ‘dirty list’ harder,” Maurício Brito, vice-coordinator for the eradication of slave labour for public prosecutors in Brasília, told the Guardian. “It will be good for those who use slave labour.”

Brito also attacked the decision to let the serving labour minister decide whether or not to publish the “dirty list”. “It stops being a juridical act and becomes a political act,” he said. Prosecutors plan to mount a legal challenge to the new rules.

In January the Inter-American Court of Human Rights ordered Brazil to pay $5m (£4.1m) to 128 former farm workers who were enslaved on a farm between 1988 and 2000. In 2003, Brazil started publishing the list of employers who kept workers in inhumane conditions.

Before the new decree, four conditions were used to categorise “slave-like labour” – being forced to work; being obliged to work to pay off debts; degrading conditions that put workers’ health or dignity at risk; an excessive workload that threatened workers’ health. Now the last two conditions only apply if workers are also forcibly kept in place – and inspections will also need a completed police report to be accepted as evidence.

Critics said the changes and increase in bureaucracy would make it harder to rescue workers from inhumane workplaces.

“The practical result is that fewer workers can be freed. There are workers in slave-labour conditions who will not be rescued,” said Leonardo Sakamoto, founder of the independent reporting group Repórter Brasil and a member of the board of trustees of the United Nations Voluntary Trust Fund on Contemporary Forms of Slavery.

Miriam Leitão, a leading columnist on the economy, attacked the move in her blog for Rio’s O Globo newspaper. “Is this really agribusiness’s agenda, at the end of the second decade of the 21st century?” she wrote. “Is it going to continue to let itself be represented as backward?”

In a statement on its website, the labour ministry said the changes aimed to “improve and give legal security” to the Brazilian state when it came to giving unemployment benefit to workers rescued from inhumane conditions and including employers on the “dirty list”. Publication of the list was suspended by the supreme court in 2014, but the government began publishing it again in March.

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“Combating slave labour is a permanent public policy of the state,” the ministry’s statement said.

Last week the ministry dismissed the chief of the division for the eradication of slave labour, André Roston. In August he had told a Senate commission that budget cuts meant it was impossible to carry out new inspections.

Critics said Temer’s government was easing off combating slave labour to cosy up to the agribusiness lobby in congress. The president is facing a vote on whether he should be suspended to face a trial on charges of racketeering and obstruction of justice. In August he won a vote on whether he should be suspended and tried on corruption charges.

“We think that the government is doing this because of big pressure of the agribusiness caucus in the national congress,” said Brito, adding that slave-like labour conditions “will increase because of impunity”.